This invention relates generally to semiconductor lasers having a multilayer structure deposited on a substrate so as to define a waveguide, wherein laser light is emitted out one end surface of the laser. More particularly, the invention relates to semiconductor lasers having single crystal mirror structures deposited directly on at least one end surface of the lasers.
In recent years, semiconductor lasers have found a number of technological applications, including optical communications systems, optical disc drives, and laser printers. Such semiconductor lasers typically are formed as a multilayer structure on a substrate, including an active layer surrounded by two cladding layers. The end surfaces of the multilayer structure, referred to herein as the xe2x80x9cfacet mirror,xe2x80x9d typically have a reflectivity in the range of 30%. Thus, mirror stacks, typically made of alternating layers of two or more amorphous materials, are applied to each facet mirror in differing thicknesses so as to adjust the reflectivity of one laser facet to about 95% and the other laser facet to about 5%. Common lasers used in the telecommunications industry are AlGaAs lasers that produce light in the 980 nm region.
Many of the applications for semiconductor lasers require operation of the lasers at high power outputs (typically above 30 mW) for extended periods of time. The operation of semiconductor lasers at high power outputs causes considerable dissipation of heat at the facet mirrors, which over time degrades these surfaces. This deterioration, termed catastrophic optical damage (COD) in the art, reduces the stability and lifetime of the semiconductor laser. Thus, the maximum power at which a semiconductor laser can be operated for extended periods of time is severely limited.
There are several known steps that can be taken to reduce the occurrence of COD. One known way to reduce the occurrence of COD is through the use of so-called xe2x80x9cwindow layers.xe2x80x9d A window layer is a layer of material, applied to the facet mirrors, which has a higher band gap than the material forming the multilayer laser structure. The window layer is largely transparent to the laser light, and thus in effect lengthens the laser cavity, thereby reducing heat buildup (and consequently COD) at the facet mirrors.
Examples of such window layers are described in Botez, D., et al., xe2x80x9cNonabsorbing Mirror (NAM) CDH-LOC Diode Lasersxe2x80x9d, Electronics Letters, Jun. 21, 1984, pp. 530-31, and in U.S. Pat. No. 5,228,047. Both of these references disclose the use of AlzGa1xe2x88x92zAs as a window layer on an AlzGa1xe2x88x92zAs multilayer structure, wherein the stoichiometry of the AlGaAs window layer is adjusted so as to give the window layer a higher band gap than that of the multilayer structure. Because the window layer does not substantially modify the reflectivity of the multilayer structure, however, it is still necessary to add a mirror stack over the window layers to obtain the necessary reflectance. Moreover, both the window layer and the mirror stack absorb laser energy, thus reducing the output of the laser and increasing heat buildup in the laser.
Another known way to reduce the occurrence of COD is through application of a passivation layer to the facet mirrors. A passivation layer is a thin layer of material, typically less than 10 nm thick, which prevent contaminants from forming on the facet mirrors. For example, U.S. Pat. No. 5,144,634 discloses the use of a passivation layer made of Si, Ge or Sb, which is applied in situ so as to prevent oxidation of the laser facets. Once the passivation material is applied, the lasers can be safely removed from vacuum and a standard mirror stack applied.
Passivation layers of the type shown in U.S. Pat. No. 5,144,634 have several drawbacks. While relatively thin, the passivation layers nonetheless absorb laser energy, causing power dissipation and heat buildup at the facet mirror. Moreover, the passivation layer has a negligible effect on the reflectivity of the facet mirrors, and so requires a full mirror stack, which again typically causes additional heat buildup and power dissipation in the laser.
U.S. Pat. No. 5,665,637 discloses an improvement over U.S. Pat. No. 5,144,634 in that polycrystalline layers of large band gap materials, such as ZnSe and ZnS, are used as a passivation layer. Because the large band gap materials do not absorb as much light energy, such passivation layers reduce the amount of heat buildup at the facet mirror. However, because the material is polycrystalline, it still absorbs some energy. Therefore, to minimize absorption, passivation layers are designed to be as thin as possible, and thus do not obviate the need for traditional mirror stacks to be applied over the passivation layers.
Accordingly, the main objective of the present invention is to provide a semiconductor laser with a single crystal mirror layer deposited directly on the mirror facet, resulting in a device having improved protection against COD and improved device characteristics, including longer lifetimes, greater power outputs, and improved stability.
The present invention achieves the aforementioned objectives by providing a semiconductor laser comprising a plurality of layers deposited on a substrate, including an active layer with neighboring cladding layers, defining a waveguide, said multilayer structure further defining opposing facet mirrors. The semiconductor laser further includes a mirror stack comprising a base mirror layer formed directly on at least one of the facet mirrors, the base mirror layer being grown epitaxially as a single crystal layer. The base mirror layer has sufficient thickness, and is made of a material having a refractive index sufficiently different from that of the multilayer structure, to substantially modify the reflectivity of the facet mirrors. The use of the single crystal mirror layer eliminates the need for either a window layer or a passivation layer, and provides improved device characteristics, including greater power output, improved stability, and an extended lifetime.
The single crystal base mirror layer is preferably made of a material which has a high band gap relative to the materials making up the multilayer structure. This further reduces the absorption of laser energy by the mirror stack.
The base mirror layer should have a thickness that is on the order of xcex/4n, where xcex is the wavelength of light that the laser is designed to operate at, and n is the refractive index of the material forming the base mirror layer. Thus, for example, for an AlGaAs communications laser designed to operate at 980 nm, the mirror layer is preferably made of a II-VI material such as ZnSe, MgS, or BeTe which is on the order of about 80-200 nm thick, and most preferably about 100 nm thick. The use of such a layer of high band gap material improves the laser""s power output and protection against COD. Moreover, the material can be epitaxially grown on the multilayer structure at relatively low temperatures, thereby avoiding unnecessary heat damage to the preprocessed laser structure.
According to the present invention, the mirror stack(s) may include additional amorphous mirror layers deposited by known techniques so as to adjust the reflectivity of the laser to the desired levels. However, in a preferred embodiment of the invention, mirror stacks are applied to both facet mirrors, and the base mirror layer of one of the mirror stacks sufficiently modifies the reflectivity of one of the facet mirrors such that no additional mirror layers are required on that surface. In the most preferred embodiment of the invention, the other mirror stack contains additional mirror layers that are also grown epitaxially as single crystal layers.
As used herein, the term xe2x80x9crefractive index sufficiently different . . . to substantially modify the reflectivity of the facet mirrorsxe2x80x9d means that the refractive index is at least 10% greater or less than that of the multilayer structure. In order to minimize the number of additional mirror layers that must be provided, it is preferred that the single crystal mirror layer have a refractive index that is at least 50% greater or less than that of multilayer structure.
A xe2x80x9csingle crystalxe2x80x9d herein means a solid which is neither polycrystalline nor amorphous. Single crystal growth possesses several signatures, including: a well-defined x-ray diffraction rocking curve with a full width at half maximum of about  less than 0.5xc2x0 and a streaked reflection high energy electron diffraction (RHEED) pattern during growth. One skilled in the art will realize that the single crystals of this invention are not limited to materials characterized by the above exemplary probing methods.
A xe2x80x9clarge band gapxe2x80x9d herein means an energy band gap at least xcx9c0.3 eV larger than that of the active layer. As many active layers have band gaps xcx9c less than 1.4 eV, the term xe2x80x9clarge band gapxe2x80x9d corresponds to at least about 1.7 eV. It is known to one skilled in the art that this group comprises numerous materials, including ZnSe, MgS, BeTe, and ternary and quaternary combinations thereof.